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Take the enhanced Section 179 deduction, which lets small and midsize businesses deduct, in 2013, up to $500,000 worth of capital equipment in the year purchased, rather than spreading the deduction out over many years.

Unless Congress extends the tax law that increased this deduction, the limit plummets to $25,000 in 2014.

A restaurant owner thinking about expanding or buying new stoves has to decide, "Do I rush and borrow some money to buy it so I can take advantage of the bigger deduction this year," or "do I roll the dice and assume Congress will extend it?" Karl says.

Zero Motorcycles, an electric-motorcycle maker in Scotts Valley, is concerned about what will happen if a federal tax credit for two- and three-wheeled plug-in vehicles, set to expire Dec. 31, is not renewed.

The credit is 10 percent of the purchase price, with a maximum credit of $2,500. Since Zero motorcycles cost about $16,000, for their buyers it's worth $1,600.

The credit expired once before - at the end of 2011 - and Zero's sales flattened out. It responded by moving into fleet sales and was successful, especially with police forces.

On Jan. 2 of this year, the tax credit was reinstated retroactively to Jan. 1, 2012. That provided a surprise windfall for people who bought a Zero cycle in 2012 and were not expecting a tax credit. It also revved up Zero's sales to consumers, but now it's facing the loss of the credit again.

Zero plans to cut prices by $1,000 next year, which should help if the credit sputters out.

The company hopes it will be extended, but "I would say the odds are against us," says Jay Friedland, the company's vice president of strategy and sustainability. "Congress is at a standstill in terms of passing anything."

Congress has always enacted temporary tax breaks to deal with short-term situations such as natural disasters or recessions or to appease certain constituents. But they have become much more common since 2000.

Most of the Bush-era tax cuts passed in 2001 and 2003 were originally set to expire after 2010.

In 2001, even though the government had been running a surplus for several years, Republicans did not have enough votes to make them permanent, says Roberton Williams, a fellow with the Tax Policy Center.

After 2001, the government never ran a surplus and passing permanent cuts got even harder.

"If policymakers decide that legislation that reduces revenues must be paid for, it is easier to find resources to offset short-term extensions rather than long-term or permanent extensions," the Congressional Research Service said in a Nov. 5 report.

During and after the financial crisis, Congress passed a raft of temporary tax measures designed to stimulate the economy.

Another reason temporary cuts are popular with lawmakers: "If you only extend it for a couple years, the lobbyists in favor of it have to keep coming back, seeking your favor. If you made it permanent, the lobbyists might go away," says Mark Luscombe, principal federal tax analyst with CCH.

When tax cuts expire, it's hard to let them go. "Not extending a tax cut is seen as increasing taxes," Williams says.

Typically, Congress bundled one-off tax breaks, like the classroom deduction, into a so-called extender package and passed it before year-end or early the next year. As more short-term tax breaks got created and extended, the package grew and grew.

By the end of 2009, there were at least 77 tax measures expiring, compared with just eight at the end of 1999.

Congress did eventually let some recession-era tax incentives go away, including the first-time home buyer credit, payroll tax cut and cash for clunkers.

When the big Bush-era tax cuts expired in 2010, most were extended until the end of 2012.

But that still left a raft of tax measures - such as the electric-motorcycle credit - expiring at the end of 2011. Rather than extend them, Congress let them expire for all of last year.

That left taxpayers wondering whether they would ever be revived, along with what would happen to breaks expiring at the end of 2012.

On Jan. 2 of this year, Congress finally passed a bill, dubbed the fiscal cliff deal. It made many Bush-era tax cuts permanent; others were modified or expired.

The deal also renewed more than 30 tax cuts that had expired at the end of 2011 and made them retroactive to the start of 2012. But most are still temporary and many face extinction on Dec. 31.

The fiscal cliff package came so late it delayed the start of tax-filing season until Jan. 30, which was 13 days later than the previous year.

Some people claiming certain tax breaks couldn't file until early March. The delay also postponed tax refunds.

The partial government shutdown in October will delay the start of this year's tax season by one or two weeks.

It's unclear whether Congress will renew soon-to-expire tax breaks or do nothing in anticipation of a big tax-reform package in 2014.

Karl says accountants know some tax breaks must be temporary but wants Congress to understand "the confusion it creates" when it passes so many of them.

Expiring tax breaks

Here are some of the 57-plus federal tax breaks set to expire Dec. 31, and what they do:

-- State and local sales tax deduction. Lets taxpayers deduct sales tax in lieu of income tax.

-- Classroom expense deduction. K-12 teachers can deduct up to $250 a year in unreimbursed classroom expenses, even if they don't itemize deductions.

-- Exclusion of cancellation of indebtedness on principal residence. Waives the federal income tax often due when recourse debt is canceled or forgiven, but only for home-acquisition mortgage on a primary residence.

-- Residential energy property credit. Gives homeowners a tax credit worth up to $500 (per lifetime) for certain energy-efficient improvements such as windows, doors, furnaces, air conditioners and water heaters. (Credit capped at $200 for windows and doors.)

-- Transit benefit parity. Lets employees use up to $245 a month in pretax salary to pay for mass transit, equal to the parking benefit. Next year, the mass transit benefit drops to $130 but parking remains at $245.

-- Race horse parity. Allows owners to depreciate racehorses 2 years old or younger over three years, the same as for older horses. Unless extended, the depreciation period for younger horses reverts to seven years in 2014.